Comb and wattle receded

[RubelliteRose]

Should I isolate her from the others?

If she isn't being picked on and if you don't feel like you need to closely monitor her, I would leave her with the others. Separation would be stressful for both her and your other chickens.
I thought about egg binding, but I would think that her comb would be red even if this was the first egg in a while. Maybe she has developed ascites?
I will tag a few of the BYC experts to see if they have any ideas for you. @Eggcessive @Wyorp Rock @azygous

 

[Eggcessive]

I agree that I would keep her with her flock, but watch out for any pecking or bullying. Some may sense that she is not well, and pick on her, but not always. Being with the flock would be the best thing for her. Tail down and the other symptoms sound like she may be having a reproductive or other disorder. Make sure that she eats and drinks daily, and check her crop in early am to make sure that her crop is emptying. Crop problems sometimes accompany reproductive illnesses. Does she have any weight loss or an enlarged lower belly? What do her droppings look like?

 

[UKchooks]

I haven't watched her pass anything, but haven't seen any unusual droppings in the coop or run. Her crop seems to be emptying fine and she's eating and drinking well. Not sure about weight as I hadn't been handling her much prior to this as she's usually very speedy and feisty! But it does look like her belly might be somewhat swollen.

 

[Wyorp Rock]

3-year-old Leghorn girls. She's off lay and her comb and wattle have receded noticeably and are paler than usual. Other than that, she seems healthy - crop normal, feet normal, droppings normal, eating well and still bossing the others about

Could worms be the issue? Should I do a worm test?

She's now sitting in a huddled posture with her tail down. Still eating plenty, but clearly not right. Please help!

She is walking more slowly than normal and kinda awkwardly, like big deliberate steps.

inspected her vent, which looks normal as far as I can tell, but looks like her abdomen might be a bit distended? But if she was egg bound, I'd have expected her to not be eating normally?

Do you have photos of her and her poop?

If she's huddling with tail down, slow and her abdomen looks distended, I'd give her 300mg Calcium Citrate+D3 now, then once daily for a week.

She may be having troubles expelling an egg, membrane or lash material.

I assume you are in UK? If possible, get a fecal float to see if she does need deworming. If a vet can't do that, then deworming may help, hard to know. The symptoms you describe line more up with reproductive issues, but worms can also take it out of an already weakened bird. In the UK you should have Panacur (Fenbendazole), Flubenvet (Flubendazole) and probably Levamisole (Prohibit) available for deworming. I *think* Flubenvet is found in your feed shops but could be wrong.

Work on hydration and provide normal feed. While you are looking her over, take note if she have any lice or mites.